This invention is related to the Thermastress process and apparatus for the die-less drawing of metal, such as, but not limited to, a low carbon steel, to obtain a predictable microstructure, tensile strength, and reduction in cross-section.
The Thermastress process is a thermo-mechanical process initially developed for producing steel and steel alloys with remarkable physical characteristics. The process differs from conventional methods for treating steel by deforming the heated steel material simultaneously with a rapid cooling step. The transformation of Austenitized steel is accomplished by an apparent shift of the critical temperature for producing Bainite (B.sub.s) brought about by the simultaneous application of stress and plastic deformation on the steel during the cooling cycle. The process inherently tends to produce Bainite rather than Martensite.
My earlier process was disclosed in U.S. Pat. No. 3,964,938 which issued June 22, 1976 for a "Method And Apparatus for Forming High Tensile Steel from Low and Medium Carbon Steel".
The basic Thermastress process involves moving material between two spaced driving means immediately adjacent heating and quenching zones. The effect of the two zones is to impose a temperature gradient on the material between the two drives so that after a gradual temperature rise, for example, to around 2,000.degree. F., the processed material is rapidly cooled.
The processed material is stretched as it passes through the heating zone, where the yield strength of the material is substantially lowered. A condition of dynamic equilibrium occurs as the material accelerates toward the downstream drive, establishing a very stable cross-section reduction profile with the cross-section of the processed material being reduced in inverse proportion to the increase in velocity. The final reduced cross-section of the material remains constant within very close dimensional tolerances.
In the case of low and medium carbon steel, the effect of a rapid cooling, as the material passes from the heating zone into the quenching zone, in conjunction with the plastic flow taking place, is to substantially modify the steel microstructure. The fine grained micro-structure, thus produced, increases the ultimate tensile strength as high as 220,000 p.s.i. and above at diameters, exceeding by a factor greater than 10, the thickness of high strength steel produced by the rapid quenching of conventional heated-finished low carbon sheet steel.
One phenomenon related to the commercial Thermastress process is that the critical temperature, at which the microstructure of steel nucleates to Bainite and Martensite, as its temperature is being reduced, shifts upwardly, compared to the conventional time temperature transformation curves for the micro-structure of such steels.
The finished microstructure of the specimen determines the ultimate strength of the material. Several factors determine the final microstructure. For example, the heating rate is important as well as the cooling rate. The velocity of the material as it passes through the heating and cooling zones is also important.
Other factors that determine the ultimate microstructure include the initial thickness or diameter of the material, the chemistry of the material, the desired finished size, thickness or diameter, the desired ultimate tensile strength as well as the tolerance range of the specimen's yield point.
Some of the problems which have prevented die-less processes from succeeding in commercial applications include variations in the longitudinal cross-section, sometimes referred to as "necks" or "eggs", and the difficulty in attaining a smooth and even surface finish.
One approach to accommodate variations in the chemistry of the steel, is to vary the amount of heat applied to the material, however, this is undesirable because changing the heat input influences the microstructure of the finished material. The cooling rate also influences the strain rate hardening rate.